Signal processing and filters for reg review ms ni

Amplifier Function and Signal Filtering used in Sleep Disorders Technology

An Overview

Midwest Sleep and Neurodiagnostic Institute, Inc

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Amplifiers and Signal Filtering

You Need to know: What differential amplifier is What CMR is and how it works How TC filters work and TC relationship to LFF How LFF and HFFs work to reduce unwanted

frequencies How a 60Hz notch filter works Know basic polarity problems The difference between an AC and DC amplifier The difference between gain and sensitivity


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Terms for Basic ElectricityImpedance: The total opposition to current flow including resistance, inductance and capacitance. Measured in ohms.Resistance:The opposition to the flow of current. Measured in ohms.Volt: Measurement of the force pushing current (electrons) through a conductor. Polarity: Refers to the positive or negative poles of a device


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Alternating Current (AC)

Alternatively flows between positive and negative (protons+ and electrons-)Alternates at 1/60 sec in USA 1/50 sec in Europe


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Ohms Law

Voltage is equal to current flowing in the circuit, multiplied by the resistance.

Voltage = E (volts)Current = I (amps)

Resistance = R (ohms)


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Voltage

Resistance

E

R

V = I x R

I ER

R EI

Current

I


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The Polygraphic Circuit

The function of the polygraph is to transform voltages generated by body into an interpretable record


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Chart Drive

- --- -- +

++

++

+

G1G2

Paper / computer


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Signal Processing

Electro conductive materialElectrode These act as a small capacitor

Wire to head boxExploring electrode C3, C4 O1, O2

Reference electrode A1, A2


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Ground

Leakage current results when there is either a break or a shunt in the circuit.Patient grounding is essential in artifact elimination and proper amplifier function.

Symbol for Ground


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Differential Amplifier

Form of AC amplifierAmplifies the difference between input (grid) 1 and input (grid) 2Rejects any similar information between G1 and G2 (CMR)


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Common Mode Rejection

Common function of differential amplifierRejection of similar activity between input 1 and input 2 Identical information rejected at near 100%


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Polarity and Localization

Input 1 When input one is negative, the pen goes

up, when input one is positive, the pen goes down.

Input 2 When input two is positive, the pen goes

up, when input two is negative, the pen goes down.


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Basic Polarity and Localization

NegativePositive

PositiveNegativeInput 1 (G1)

Input 2 (G2)

Input 1 rules

Input 2 rules

Up rules Down rules


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Common Mode Rejection Ratio

Common Input Signal Output Signal

Should be high ie. 100,000 to 1

AC coupled Amplifier


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Polarity Calculation

26 uv

-15 uv

26

- 15

+ 41 uv

+ 41 uv


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Galvanometer

Limited to a maximum response of 90 - 120hz


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Amplifier/Machine Calibration

All filters set the same


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Montage Calibration


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Time Axis

The point (time) at which each pen falls along the same recording line

The galvonometer or pen mount may be adjusted to correct problems

Not a problem with digital systems


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Damping

Adjustment to reduce overshootReduces erroneous fast-wave artifactOnly adjusted on polygraphs Not digital systems

good

Under damping

Over damping


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Mechanical Baseline

Pen spacing withoutamplifier input to

pens.


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Electrical Baseline

The “electrical zero” of a recording pen position

Found by turning the individual chart amplifier power off and on while adjusting the baseline knob.

EZ if found when the pen stops changing positions between power-on, and power-off

Electrical Zero

Mechanical Baseline needs adjustment


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1. Unequal sensitivity

2. Pen alignment—time axis

3. Pen baseline off

4. LFF different setting


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Gain vs. Sensitivity

Gain is a measure of the ability to change the magnitude of the input voltageSensitivity is the amount of output (pen deflection) to a given input voltage


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Voltage

Pen DeflectionSensitivity

S

V

PD

V = S x PD

S VPD

PD VS

Sensitivity

S

uV

uV/mmmm


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Sensitivity Calculations

Sensitivity is at 5uv/mm the voltage is 50 uv what is the deflection?

D = V/S

D = 5uv/mm 50uv

D = 10 mm


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Sensitivity is set at 7 uv/mm and the deflection is 12 mm. What is the voltage?V = S X D

V = 7uv/mm X 11 mm

V = 77 uv


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Voltage is 75uv and the deflection is 15 mm. What is the Sensitivity setting?

S = V/D

S = 15mm 75 uv

S = 5 uv/mm


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60 Hertz Notch Filter

Also known as an AC FilterIn North America, all AC electrical current oscillates at 60 Hz.Uses CMR by tapping into line current and measuring it against output.


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0

10

2030

40

50

60

7080

90

100

40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80

60 Hertz Notch Filter


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Units of Measure Used in Filtering

Hertz (Hz) Most common unit in electroneurodiagnostics

Second (sec) Used in Time Constant (TC) filter calculations

Decibels (dbl) Most basic unit of measure in frequency filtering Rarely referred to in sleep diagnostics


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Time Constant (TC)

Units = seconds (s)The time it takes in seconds for a waveform to drop to

37% of its calculated amplitude

Faster TC – Reduces amp. of slow frequencies

Low cut – high pass filter

100%

37%

Time (s).3.003 1


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TC (sec) and LFF (Hz) Relationship

TC and LFF work with same goal Reduction of slow frequencies Different units of measure to describe the same

action

Conversions:

1

2π x LFF TC =

1

2π x TCLFF =


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Basics of LFF and HFFs

Reduces amplitude of listed frequency at a fixed value 20% (Nihon Kohden) 30% (Grass)

Reduces amplitude of frequencies above (HFF), or below (LFF) at linear levels


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Low Frequency Filters

Designed to reduce the amplitude of frequencies at and below the selected frequency “Low cut, high pass filters”.


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Expected Amplitude Decay: LFF = 10HzBased on Nihon Kohden filters

Reduces amplitude of a 10 Hz signal by 20% Reduces a 5 Hz signal by 50%

Reduces a 2.5Hz signal by 75% Reduces a 1.25 Hz signal by 87.5%

0

10

20

30

40

50

60

70

80

90

100

Am

pli

tud

e

1.25Hz 2.5Hz 5Hz 10Hz


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Low Frequency Response Curve


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High Frequency Filters

Designed to reduce the amplitude of frequencies at and above the selected frequencyHigh cut – low pass filters


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Reduces amplitude of a 70Hz signal by 20% Reduces a 140 Hz signal by 50%

Reduces a 280 Hz signal by 75% Reduces a 560 Hz signal by 87.5%

Expected Amplitude Decay: HFF = 70Hz(based on Nihon Kohden filters)

0

10

20

30

40

50

60

70

80

90

100

Am

pli

tud

e

70Hz 140Hz 280Hz 560Hz


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High Frequency Response Curve


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Filters Effect Amplitude & Phase

Note: The Higher the filter the lower the amplitude

Digital Filters will not cause a Phase Shift

High Filter

Low Filter


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Direct Current Amplifiers

Used to amplify signals which represent either all negative, or all positive voltage Not alternating between – and + (AC)Used most often with slow trending data (SaO2 and heart rate trending)No LFF or HFF filters utilized TC infinite (No frequencies to filter)


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Wheatstone Bridge Circuit

A four-arm bridge used to measure resistance in a circuit Commonly used in DC amplifiers when resistance devices are in use Strain gauges

NPT Effort gauges


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Analog to Digital Conversion

Brainwaves and other PSG signals are analog or continuousDigital is based on values of 0 or 1ADC samples the signal and assigns a digital value


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Sampling Rate

The rate at which the computer program samples the analog information to convert it to digital signal. The faster the sampling rate the more

accurate the tracing is The faster the sampling rate the larger the

patient data file will be


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Sampling Rate

Nyquist principle The sampling rate must be at least twice

as fast as the fastest frequency that will be recorded.

ASET Standard is three times faster than the HFF setting

Can vary by channel 200+/sec for EEG 10/sec for respiratory


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Sampling Rate

Aliasing Sampling rate too low

Signal processing and filters for reg review ms ni

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